dart:io file api doesn't work with long file names on windows

[sigurdm]

Dart version: Dart VM version: 2.8.4 (stable) (Wed Jun 3 12:26:04 2020 +0200) on "windows_x64"
Windows 10 Enterprise

Reproduction:

import 'dart:io';

void main() {
 File('${'nana' * 100}').writeAsStringSync('hi');
}

Prints:

Unhandled exception:
FileSystemException: Cannot open file, path = 'nananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananana' (OS Error: The system cannot find the path specified.
, errno = 3)
#0      _File.throwIfError (dart:io/file_impl.dart:645:7)
#1      _File.openSync (dart:io/file_impl.dart:489:5)
#2      _File.writeAsBytesSync (dart:io/file_impl.dart:614:31)
#3      _File.writeAsStringSync (dart:io/file_impl.dart:638:5)
#4      main (file:///C:/Users/sigurdm/pana/bin/long_names.dart:5:25)
#5      _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:301:19)
#6      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)

The program works if the name is shorter.

The same happens if I try deleting a directory containing files with long names with Directory.delete(recursive:true);. (That's how I stumbled into this).

According to https://docs.microsoft.com/en-us/windows/win32/fileio/naming-a-file#maximum-path-length-limitation there should be ways of enabling long paths on windows 10.

[zichangg]

I'm looking at the APIs and try to add to our VM.

I think your example is not going to work. The file name itself (4 * 100) has exceeded the limit.

This type of path is composed of components separated by backslashes, each up to the value returned in the lpMaximumComponentLength parameter of the GetVolumeInformation function (this value is commonly 255 characters).

@sigurdm I'm trying to experiment on my Windows machine. How did you enable the "long path"? I've modified the registry key and reboot my machine. But I'm still not able to create a file with path longer than MAX_PATH.

[sigurdm]

If you clone https://github.com/dart-lang/pana and run pub run test/end2end_test.dart it creates a filename too long to delete for dart (it will fail when deleting the temporary pub-cache from tearDownAll).
I believe that file is created by pub untar-ing files with too long names inside. (it uses 7z as a sub-process to untar on windows).

[zichangg]

https://dart-review.googlesource.com/c/sdk/+/152736 is working in progress, which will enable the long path on Windows.

One problem with cl is Directory.current doesn't work with long path.

Another solution can be added in the future is to embed the manifest into dart.exe which enables long path. The drawback of this solution is that this isn't supported on earlier versions of Windows.